Pump or motor device



Jan. 30, 1968 s. s. BAITS ET AL 3,366,072

PUMP QR MO TOR DEVICE Filed Nov. 1964 2 Sheets-Sheet l Jan. 30, 1968 s.s. BAITS ET AL 3,366,072

PUMP (JR-MOTOR DEVICE Filed Nov. 5, 1964 2 Sheets-Sheet 2 if H UnitedStates Patent O 3,366,072 PUMP R MOTOR DEVICE Stephen S. Baits andWalter J. Iseman, Rockford, Ill., illSlS lgIlPl'S to SundstrandCorporation, a corporation of Filed Nov. 5, 1964, Ser. No. 409,130 12Claims. (Cl. 103162) The present invention relates to hydraulic energytranslating devices and more specifically to axial piston pumps andmotors wherein valving is accomplished by rotation of a cylinder blockagainst a stationary valve member.

In axial piston units of the type described, an angularly related cammember reciprocates the pistons in a rotating cylinder block and thepistons receive and discharge fluid through arcuately shaped ports in astationary valve plate engaging the cylinder block. In existing units,optimum speeds for obtaining high poWer-to-weight ratios with goodefflciency are about 120 inches per second average piston velocity. Itis desirable that the unit be able to run faster than this in order towithstand overspeeds or to obtain higher power-to-Weight ratios.

So-called slipper units, with individual slippers on the pistonsengaging the cam plate, can attain 120 inches per second, but only withvery careful development and heavy spring bias on the pistons andcylinder block urging the former toward the cam and the latter towardthe valve plate. One difliculty lies in the tendency of the slippers totip off the cam plate due to centrifugal force. Slippers sufferimmediate permanent damage if tipping occurs. Another difliculty lies inthe high tipping moment on the block resulting from the high centrifugalforces of the relatively heavy pistons. Further, the inherent overhangof the pistons in slipper units is distinctly detrimental to performancebecause of the side loads on the pistons.

So-called bent units perform well but are bulky and awkward in variablestroke designs where the entire cylinder block and valve plate arepivotal to vary stroke.

It is therefore a primary object of the present invention to provide anew and improved axial piston pump with no overhung loads on the pistonsand stroke adjustability by tilting the cam plate.

It is also an object of the present invention to provide a new andimproved axial piston pump with articulated pistons connected to theswashplate through a unitary annular ring having only axial forcesthereon to readily permit high speed operation.

Another object of the present invention is to provide a new and improvedaxial piston pump with connecting rods between the swashplate and thepistons with the connecting rods substantially perpendicular to theswashplate in all adjusted positions thereof so that the forces on theswashplate are substantially perpendicular only,

A further object of the present invention is to provide a compactvariable stroke pump with lightweight pistons and low inherent clearancevolume allowing eflicient operation for long life at very high speeds.

Another object of the present invention is to provide a new and improvedaxial piston pump with a cylinder block having a length substantiallyshorter than its diameter and the piston forces acting laterally on thecylinder block closely adjacent to the valve plate.

A more specific object of the present invention is to provide a new andimproved axial piston fluid pump having a housing with a valve platemounted therein with inlet and outlet passages and having a portingsurface thereon, a cylinder block rotatably mounted in the housinghaving a plurality of axial cylinders and a face communicating with thecylinders and engaging the valve plate surface, the block having anaxial length approximately the same as its radius, an input shaftrotatably mounted in the housing and splined to the cylinder block forrotating the same,

pistons slidably mounted in the cylinders adapted to receive and expelfluid through the passages, the pistons having semispherical socketstherein, a swashplate pivotally mounted in the housing and having a flatcamming surface, a unitary annular ring rotatably mounted on the cammingsurface with a plurality of semispherical sockets therein correspondingin number to the pistons, a plurality of connecting rods between thering and the pistons each having semispherical balls at both endsthereof seated respectively in one of said piston sockets and one of thering sockets, means to restrain the connecting rods to stayapproximately perpendicular to the cam plate in all adjusted positionsof the cam member, and a resiliently biased retaining ring for urgingthe connecting rods and the annular ring against the camming surface andfor urging the cylinder block against the valve plate.

Other and further objects and advantages of the present invention willbe readily apparent from the following detailed description taken inconnection with the accompanying drawings, in which:

FIG. 1 is a longitudinal sectional view of the hydraulic unit of thepresent invention;

FIG. 2 is a cross-sectional view of the hydraulic unit taken generallyalong line 22 of FIG. 1;

FIG. 3 is a cross-sectional view taken generally along line 3-3 of FIG.1 with the swashplate and connecting rods removed.

While this invention is susceptible of embodiment in many differentforms, there is shown in the drawings and will herein be described indetail an embodiment of the invention with the understanding that thepresent disclosure is to be considered as an exemplification of theprinciples of the invention and is not intended to limit the inventionto the embodiment illustrated. The scope of the invention will bepointed out in the appended claims.

It should be understood that, while the hydraulic unit described in thisspecification is referred to as a pump, the principles of the inventionare equally applicable to a hydraulic unit acting as a motor.

Referring now to FIG. 1, a pump housing 10 may c0mprise a unitary met-a1casting having a bushing 11 pressfitted within a counterbore in one endof the housing 10 for seating the outer race of bearing 12. The bearing12 supports a pump input shaft 13 Within the housing 10. One end ofshaft 13 has splines 14 thereon which engage drive splines in a centralbore 16 in a pump cylinder block 17.

The cylinder block 17 is a cylindrical metal body having, for example,seven cylinders 18 formed therein opening directly to end face 20 of thecylinder block and communicating with the cylinder block port face 21through cylinder block ports 22. Viewing FIG. 3, radially inwardly ofcylinders 18 and angularly disposed intermediate the cylinders, closedend bores 25 are formed in an annular pattern in the cylinder block 17opening at cylinder block face 20 for receiving springs for a purposedescribed hereinafter. As shown in FIG. 1, the axial length of thecylinder block 17 along the axis of rotation Otf shaft 13 is less thanthe radius of the block in a plane perpendicular to the axis of shaft13.

Slidably mounted in each of the cylinders 18 are pistons 26 each havinga semispherical socket 27 which surrounds and captures a spherical ball28 of one of a pinrality of connecting rods 29. Spherical balls 31 areintegrally formed on the other ends of the connecting rods 29respectively, and are seated within semispherical sockets 32 in aunitary annular ring 33. The connecting rods 29 are therefore pivotallyconnected to both the pistons 26 and the unitary annular ring 33.

The annular ring 33 serves to transfer the camming force from theswashplate or cam member 35 to the pistons 26 and has the plurality ofthe sockets 32 formed in one face thereon a circle drawn about the axisor center of the annular ring 33. The swashplate or cam member 35 ispivotally mounted in the housing by rollers 40 and 41 rotatably mountedon shafts 42 and 43 fixed in the housing 10, there being two rollers oneach of the shafts 42 and 43. Parallel arouate surfaces 45 are formed onthe back of the swashplate 35 to receive rollers 40 and 41 so that theswashplate pivots about point 46. As shown, a counterbore is formed inthe face of the swashplate 35 loosely receiving annular ring 33 anddefining a camming surface 48 which serves to earn the unitary annularring 33 and :the pistons 26 to drive the pistons to the right and expelfluid from the cylinders 18.

In this specific construction illustrated, the swashplate 35 ispivotable from the maximum stroke position illustrated in FIG. 1 to aneutral, no-stroke position in which the cam face 48 is parallel to theend of the cylinder block, but it will be understood that in otherembodiments, the swashplate may be fixed or pivotable in oppositedirections from a neutral, no-stroke position. The center 46 about whichthe swashplate pivots is usually located near the point where the shaftaxis pierces the plane containing the spherical connections of rod ends28 with pistons 26. The pivot axis 46 may be located directly at suchpoint, but it is preferably displaced from such point herein, axiallyand radially, in order to modify the nature of the forces required tocontrol the swashplate angle, and such relocation of the pivot isfacilitated by the present constnuction including the free mounting ofring 33 in the swashplate.

The cylinder block 17 is supported by a bearing 49 in the housing 10,and the point where the shaft axis pierces the plane containing thespherical rod ends 28 preferably bisects the axial length of the bearing49 supporting the cylinder block in the housing 10, or, in the case of ashaft supported block, bisects the radially locating connection betweenblock and shaft.

In order to cause rotation of the annular ring 33, with the cylinderblock 17, the connecting rods 29 are maintained approximatelyperpendicular to the annular ring 33 by means of a retaining ring 50fixed on the annular ring 33 and disposed inside the annular array ofconnecting rods 29. The retaining ring 50 is formed with guide channelsas at 50a respectively, receiving the reduced central portions of theconnecting rods 29 and maintaining the latter generally perpendicular tothe ring 33 but permitting limited pivotal movement relative to the ring33 as will appear.

At a full displacement position of the swash-plate member 35 theconnecting rods have a substantial angular relationship to the pistons26 and therefore exert side loads on the pistons through the sphericalballs 28, but the connecting rods 29 remain substantially perpendicularto the camming surface 48 in all adjusted positions of the swashplate 35and therefore exert no side loads, or loads parallel to face surface 48on the cam member or swashplate 35. As the input shaft 13 rotates thecylinder block 17 together with the pistons 26 and the annular ring 33,the pistons 26 and the universal connections 28 therein will travel in acircular path about the axis of the shaft 13, and the universalconnections 31 in the annular ring 33 will travel in a circular pathabout the axis of the ring 33, but when viewed from the plane of theannular ring 33, the connections 28 will travel in an elliptical pathrelative to the path of the connections 31 so that while the connectingrods remain effectively normal ot the swashplate, some limited pivotalmovement must be provided at the ball joints 31 and the fit of theconnecting rods in the guide channels 50a permits the necessarymovement. As the annular ring 33 holding the connecting rods in placeagainst the swashplate 35 is a one piece construction, the centrifugalforces on the rods due to rotation of the ring with the cylinder block17 are restrained and there is no danger of the rods being thrownradially outward as in separate slipper units.

Each of the connecting rods 29 exerts a lateral force on the pistons 27,the centroid of which substantially bisects the axial length of thecylinder block bearing 49 as described above, and lies close to the portface 21 a distance therefrom less than the radius of the block. In thismanner the side forces on the block 17 are located close to the portingface 21 of the cylinder block and are effectively opposed by bearing 49,the proper location of which is facilitated by the present construction.

The retaining ring 50 is mounted within a central opening 51 of theannular ring 33 and engages the annular ring and the spherical balls 31on the connecting rods 29 thereby serving as a piston return mechanismholding the connecting rods and the annular ring 33 against theswashplate 35 during intake strokes of the piston, i.e., when thepistons are moving to the left in FIG. 1. The retaining ring 50 isresiliently urged toward the annular ring 33 by springs 52 mounted ineach of the closed end bores 25 of the cylinder block acting throughmeans including an annular spring seat member 54a of angular crosssection slidably fitted on shaft 13 tightly enough to preventobjectionable tipping and carrying an annular semispherical ball member54b loosely mounted on the member 54a. The spherically shaped outersurface of member 5412 engages a complementary inner surface of retainer50, both preferably concentric about point 46. In this manner thesprings 52 also serve to urge the cylinder block 17 against a portingand seating surface 60 on valve plate 61 which is bolted to housing 10as at 62. The cylinder block 17, connecting rods 29, and annular ring 33rotate as a unit with the input shaft 13.

The port plate 61 has arcuate ports 64 and 65 opening to the seatingsurface 60 that serve as either high or low pressure ports dependingupon the direction of rotation of input shaft 13 and operation as a pumpor motor. Communicating with the ports 64 and 65 are inlet or outletpassages 66 and 67 in the port plate 61 adapted to be connectedrespectively to a suitable Source of fluid (not shown) and the hydraulicload (not shown).

The angle of the swashplate may be controlled in any suitable manner. Asillustrated in FIG. 2, a constant pressure valve 70 is mounted withinthe port plate 61 and serves to maintain a constant outlet pumppressure. As the details of the constant pressure valve form no part ofthe present invention, they will be described only generally. A chamber71 communicates with the high pressure passage of the pump normallybalancing spool 72 against the force of spring 73 in operation. As thepressure in high pressure passage increases above the desired constantlevel, the spool 72 will move to the right against the force of thespring, closing port 74 connected to the tank (not shown) and openingport 75 which is also connected to the high pressure passage (or asource of control fluid) thereby admitting pressure fluid to passage 76.Passage 76 communicates with a chamber 78 behind the swashplate controlpiston 79 shown in FIG. 1. In this manner as the pressure increases thepiston 79 will move to the left decreasing the angle of the swashplate35 and reducing the piston stroke and thereby lowering the pressure inthe high pressure passage. When the pressure in the high pressurepassage again reaches the desired level, the pressure in chamber 71 willbalance the force of the spring 73 on the spool valve and hold thepiston 79 at a position to deliver fluid at the desired pressure.

A swashplate return mechanism 80 pivots the swashplate or cam member 35to full displacement when the spool valve ports the chamber 78 to tank.The return mechanism consists of a piston 81 urged toward the swashplateby a spring 82 mounted within sleeve 83 fixed within a bore in a boss 85formed on the upper portion of the housing 10.

An end cap 86 bolted to the left end of housing 10 as at 87 supports ashaft seal assembly generally designated by the numeral 88. The shaftseal consists of a first seal member 89 fitted on shaft 13 and a secondseal member 90 mounted within a counterbore in end cap 86 andresiliently urged toward the first seal member 89 by springs 91. Sealmembers 89 and 90 engage one another along radial surfaces 92 and 93forming a seal preventing the escape of any hydraulic fluid from theinterior of the pump housing. Suitable seals 95 and 96 prevent theescape of fluid along the housing and along the shaft 13 respectively,and a seal 97 prevents leakage between the end closure and the housing.

In order to hydrostatically balance the ring 33 and reduce frictionbetween the ring and the swashplate face 48, thereby to reduce wear, theback face of the ring 33 is preferably provided with fluid chambers 100respectively ported as at 101 to the sockets receiving the balls 31, andeach of the connecting rods 29 includes a longitudinal passage 102communicating with a port 103 through the end of the associated piston26 and leading to the cylinder in which the piston reciprocates.

Summarizing the advantages of the construction described herein, it willbe understood that use is made of relatively short pistons which arelight in Weight and re- Sult in low block tipping moment fromcentrifugal force and low piston return inertia forces. Clearance volumeis inherently low. As a result of the reduced tipping moment, a moreuniform film thickness is maintained between the block and the portplate. The pistons are free from overhung loads, and the ball and socketconnection of the rod to the piston allows the piston to conform to thebore thereby to utilize the entire piston area to absorb the side load.The centroid of the piston side loads is located less than 75 percent ofthe length of the block from the cylinder block port face and iseffectively opposed by means mounting the cylinder block. Variablestroke and displacement are obtained without swinging the cylinderblock. The angle of the connecting rods relative to the pistons isvariable with variation in stroke, but the connecting rods remainessentially normal to the swashplate face regardless of stroke, and arerestrained in a unitary ring allowing for very high speed operation andfreedom in placement of the cam plate pivot point. The cylinder blockmay be short and lightweight. The compact construction results insavings in space, weight, and manufacturing costs. Piston speeds can bein excess of 120 inches per second easily, and this is useful inproviding overspeed capability and in obtaining very exceptionalpower-to-weight and size ratios.

We claim:

1. An energy translating device comprising: a valve plate having inletand outlet ports therein and having a seating surface, a rotatablecylinder block having a plurality of cylinders therein and having a facecommunicating with said cylinders, said face engaging said seatingsurface and rotatable with respect to said valve plate, said blockhaving an axial length substantially less than its diametrical width,means locating said cylinder block radially, pistons slidably mounted insaid cylinders adapted to receive and expel fluid through said passages,a cam member having an camming surface for reciprocating said pistons,and rigid rods pivotally connected at one end to said camming surfaceand pivotally connected at their other ends to said pistons toreciprocate said pistons, said rods being substantially perpendicular tosaid camming surface, said rods exerting a side force on each of saidpistons, the centroid of the points of pivotal interconnections betweensaid rods and said pistons lying in a plane perpendicular to the axis ofsaid cylinder block and bisecting said locating means.

2. An axial piston fluid pump comprising: a housing, a valve platemounted on said housing having inlet and outlet passages therein havinga seating surface thereon, a cylinder block rotatably mounted on saidhousing and having a plurality of axial cylinders therein and having aface communicating with said cylinders and engaging said valve platesurface, said block having an axial length approximately the same as itsradius, an input shaft ro- 6 tatably mounted in said housing and splinedto said cylinder block for rotation thereof, pistons slidably mounted insaid cylinders adapted to receive and expel fluid through said passages,said pistons having semisphen'cal sockets therein, a cam memberpivotally mounted in said housing and having a flat camming surface, aunitary annular ring rotatably mounted on said camming surface, saidring having a plurality of semispherical sockets therein correspondingto said pistons, a plurality of rigid connecting rods between said ringand said pistons each having semispherical balls at both ends thereofseated respectively in one of said piston sockets and one of said ringsockets, said rods 'being substantially perpendicular to said cammingsurface in all adjusted positions of said cam member, said rods exertingthrough said semispherical balls lateral forces on said pistons, thecentroid of the points of origin of the forces lying within saidcylinder block relatively close to said seating surface, said cam memberpivotal axis passing approximately through the centroid of said plane,and a resiliently biased retaining ring for urging the connecting rodsand the annular ring against the camming surface and for urging thecylinder block against the valve plate said retaining ring includingmeans for rotating said rods with the cylinder block.

3. An axial piston fluid pump having a housing, a valve plate mountedtherein with inlet and outlet passages and having a porting surfacethereon, a cylinder block rotatably mounted in the housing having aplurality of axial cylinders and a face communicating with the cylindersand engaging the valve plate porting surface, the block having an axiallength approximately the same as its radius, an input shaft rotatablymounted in the housing and splined to the cylinder block for rotatingthe same, pistons slidably mounted in the cylinders adapted to receiveand expel fluid through the passages, the pistons having semisphericalsockets therein, a swashplate pivotally mounted in the housing andhaving a fiat camming surface, a unitary annular ring rotatably mountedon the camming surface with a plurality of hemispherical sockets thereincorresponding in number to the pistons, a plurality of rigid connectingrods between the ring and the pistons each having semispherical balls atboth ends thereof seated respectively in one of said piston sockets andone of the ring sockets, means constraining the connecting rodsapproximately perpendicular to the camming surface in all adjustedpositions of the cam member, and a resiliently biased retaining ring forurging the connecting rods and the annular ring against the cammingsurface and for urging the cylinder block against the valve plate.

4. An axial piston pump comprising: a housing, a valve plate in thehousing having inlet and outlet passages, a cylinder block rotatablymounted in the housing and having a plurality of axial cylinderscommunicable through one end of the block successively with said inletand outlet passages on rotation of the cylinder 'block, an input shaftrotatably mounted in the housing and connected to the cylinder block forrotating the latter, pistons reciprocable in the cylinders respectively,a swashplate mounted in the housing at the opposite end of the cylinderblock and having an inclined camming surface, annularly arranged bearingmeans rotatably mounted and acting as a unit on the camming surface,said annularly arranged bearing means being unrestrained peripherally, aplurality of rigid connecting rods having opposite ends respectivelypivotally connected to the pistons and to the annularly arranged bearingmeans, means on the annularly arranged bearing means permitting limitedpivotal movement of the rods relative to the ring and constraining therods to remain approximately perpendicular to the camming surface, saidlast recited means laterally locating said annularly arranged bearingmeans.

5. An axial piston pump, comprising: a housing, a valve plate in thehousing having inlet and outlet passages, a cylinder block rotatablymounted in the housing and having a plurality of axial cylinderscommunicable through one end of the block successively with said inletand outlet passages on rotation of the cylinder block, an input shaftrotatably mounted in the housing and connected to the cylinder block forrotating the latter, pistons reciprocable in the cylinders respectively,a swashplate mounted in the housing at the opposite end of the cylinderblock and having an inclined camming surface, an annular ring rotatablymounted on the camming surface with freedom to move laterally, aplurality of rigid connecting rods having opposite ends respectivelypivotally connected to the pistons and to the annular ring, means on theannular ring permitting limited pivotal movement of the rods relative tothe ring and constraining the rods to remain approximatelyperpendicularly to the camming surface, said last recited meanslaterally locating said annular ring, a spherically shaped inner surfaceon said annular ring, and a ball member on said input shaft having aspherically shaped outer surface engaging the spherically shaped innersurface on said annular ring, and spring means acting against saidcylinder block and said last recited member, urging the cylinder blocktoward the valve plate and urging the annular ring toward the cammingsurface.

6. A combination as defined in claim 5, wherein the ball member isloosely mounted around said shaft for limited lateral movement relativethereto.

7. A combination as defined in claim including means mounting theswashplates for pivotal movement about an axis passing through thecenter of the spherical surface on said ball member.

8. A combination as defined in claim 5 including a spring seat memberslidable on said shaft and having an angular cross section including aradially outwardly extending annular flange engaging said spring meansand an axially extending annular portion on said shaft and looselycarrying said ball member.

9. A hydraulic energy translating device comprising: a valve platehaving inlet and outlet ports therein and having a seating surface, acylinder block having a plurality of cylinders therein and having a faceslidably engaging said seating surface, pistons slidably mounted in saidcylinders adapted to receive and expel fluid through said ports, a cammember having a camming surface for reciprocating said pistons, meanspivotally mounting said cam member for varying the displacement of thedevice, annularly arranged bearing means rotatably mounted and acting asa unit on said camming surface with a plurality of semispherical socketstherein corresponding in number to the pistons, a plurality of rigidconnecting rods between the bearing means and the pistons each havingsemispherical portions at both ends thereof seated respectively in oneof said pistons and one of the bearing means sockets, and meansengageable with said rods separate from said bearing means sockets fordriving said bearing means whereby timing torque for the bearing meansis transferred from the cylinder block through the rods to the bearingmeans.

10. A hydraulic energy translating device as defined in claim 9 whereinsaid means engageable with said rods permits limited pivotal movement ofthe rods relative to the bearing means and constrains the rods to remainapproximately perpendicular to the camming surface, said last recitedmeans laterally locating said bearing means, said cam memberunrestraining said bearing means radially.

11. An axial piston pump as defined in claim 4 wherein said means on thebearing means permitting limited pivotal movement of the rods relativeto the bearing means includes guide means engageable with said rods toassist in driving the bearing means in rotation.

12. A hydraulic energy translating device comprising: a housing member,a valve member having inlet and outlet ports therein, a cylinder blockrotatably mounted in said housing member and having a plurality ofcylinders therein, said block being in engagement with the valve memberand rotatable relative to said valve member, a bearing supporting saidcylinder block and aligned in a plane passing through said cylinders,said bearing comprising a roller bearing mounted in said housing memberand having a portion thereof engaging the periphery of said cylinderblock, pistons slidably mounted in said cylinders, a cam having acamming surface adjacent the other end of said block, and a plurality ofrigid rods connecting the cam to said pistons, each having a universalconnection with said cam and with one of said pistons, the centers ofthe universal connections with the pistons defining a plane the centroidof which substantially bisects said bearing.

References Cited UNITED STATES PATENTS 1,710,567 4/1929 Carey 1031621,908,612 5/1933 Johnson 103-162 2,141,935 12/1938 Rose 103162 2,146,1172/1939 Gros 103162 2,860,581 11/1958 Buchner 103162 2,967,491 1/1961Wiggermann 103l62 3,036,434 5/1962 Mark 103-162 3,075,472 1/1963 Garnier103162 3,096,723 7/1963 Puryear 103162 FOREIGN PATENTS 539,637 9/1941Great Britain.

ROBERT A. OLEARY, Primary Examiner.

MARK M. NEWMAN, DONLEY J. STOCKING,

Examiners.

R. M. VARGO, W. L. FREEH, Assistant Examiners.

Patent No. 3,366,072 January 30, 1968 Stephens S. Baits et al.

ed that error appears in the above identified It is certifi e herebycorrected as patent and that said Letters Patent ar shown below:

In the heading to the printed specification, lines 4 and 5, "acorporation of Illinois" should read a corporation of Delaware Signedand sealed this lOth day of March 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

1. AN ENERGY TRANSLATING DEVICE COMPRISING: A VALVE PLATE HAVING INLETAND OUTLET PORTS THEREIN HAVING A SEATING SURFACE, A ROTATABLE CYLINDERBLOCK HAVING A PLURALITY OF CYLINDERS THEREIN AND HAVING A FACECOMMUNICATING WITH SAID CYLINDERS, SAID FACE ENGAGING SAID SEATINGSURFACE AND ROTATABLE WITH RESPECT TO SAID VALVE PLATE, SAID BLOCKHAVING AN AXIAL LENGTH SUBSTANTIALLY LESS THAN ITS DIAMETRICAL WIDTH,MEANS LOCATING SAID CYLINDER BLOCK RADIALLY, PISTONS SLIDABLY MOUNTED INSAID CYLINDERS ADAPTED TO RECEIVE AND EXPEL FLUID THROUGH SAID PASSAGES,A CAM MEMBER HAVING AN CAMMING SURFACE FOR RECIPROCATING SAID PISTONS,AND RIGID RODS PIVOTALLY CONNECTED AT ONE END TO SAID CAMMING SURFACEAND PIVOTALLY CONNECTED AT THEIR OTHER ENDS TO SAID PISTONS TORECIPROCATE SAID PISTONS, SAID RODS BEING SUBSTANTIALLY PERPENDICULAR TOSAID CAMMING SURFACE, SAID RODS EXERTING A SIDE FORCE ON EACH OF SAIDPISTONS, THE CENTROID OF THE POINTS OF PIVOTAL INTERCONNECTIONS BETWEENSAID RODS AND SAID PISTONS LYING IN A PLANE PERPENDICULAR TO THE AXIS OFSAID CYLINDER BLOCK AND BISECTING SAID LOCATING MEANS.